1. Field of the Invention
This invention relates to the field of making mixed metal hydroxides or layered double hydroxide compounds. More specifically, the invention relates to an improved synthetic meixnerite product and method for making the same.
2. Technology Review
Naturally occurring meixnerite exists as a secondly mineral in the cracks of serpentine rocks near Ybbs--Persenberg in lower Austria. In its crystalline state, such meixnerite material is tabular, colorless and has perfect basal cleavage. Natural meixnerite is closely related to hydrotalcite and pyroaurite in overall structure. Its infrared absorption spectrum compares favorably to those for hydrotalcite and other synthetic magnesium-aluminum double hydroxides. In some circles, meixnerite is even listed among other hydrotalcite-like materials, or grouped in the broader family of "hydrotalcites". Under the latter definition, meixnerite is a carbonate-free member of the hydrotalcite family which has only hydroxy anions. Still others refer to meixnerite as an all hydroxyl, layered double hydroxide.
Meixnerite, or magnesium aluminum hydroxide hydrate, is often symbolized by the formula Mg.sub.6 Al.sub.2 (OH).sub.18.4H.sub.2 O, although still other formulaic representations include: Mg.sub.4 Al.sub.2 (OH).sub.14.3H.sub.2 O and Mg.sub.3 Al(OH).sub.8 !OH.2H.sub.2 O.
While the synthesis of meixnerite is fairly new, these various methods of manufacture do not appear to be commonly practiced or commercially practical. In March 1980, G. Mascolo et al. described a synthesis process in Mineralogical Magazine whereby magnesium oxide, decomposed from basic magnesium carbonate at 650.degree. C. for 6 hours, was combined with an alumina gel and rotated in an air thermostated oven for one week at 80.degree. C. The resulting product was then dried over silica gel. It was analyzed to contain some brucite compound and about 0.8-1.0 wt. % carbon dioxide.
Six years later, I. Pausch et al. wrote of a variation on the aforementioned process in Clay and Clay Minerals. Therein, magnesium oxide, annealed at 1050.degree. C., was combined with an alumina gel (.delta.--Al.sub.2 O), MgC.sub.2 O.sub.4.2H.sub.2 O and distilled water. This combination was heated to between 100.degree.-350.degree. C. at a pressure of 100 MPa for various reaction times ranging from 7 to 42 days. IR spectroscopy analysis of the resulting product showed some carbonate contamination, but at an intensity of less than 5% as compared to natural hydrotalcite.
From a series of experiments reported by E. Dimotakis et al. in Inorganic Chemistry, vol. 29, No. 13 (1990), synthetic meixnerite was prepared by calcining a hydrotalcite of the formula Mg.sub.3 Al(OH).sub.8 ! CO.sub.3 !.sub.0.5.2H.sub.2 O at 500.degree. C. to form a metal oxide solution. This oxide was then hydrolyzed at 25.degree. C. in a carbon dioxide-free environment.
It is a principal objective of this invention to provide an improved means for making synthetic meixnerite. It is another objective to provide a process for synthesizing meixnerite and related minerals from two or more powders. It is still another objective to cream a hydrotalcite-like compound having significantly lower carbonate levels and virtually no other anion contamination. It is still another objective to provide a method for making synthetic meixnerite which is not dependent on the use of alumina gels. It is still another objective to make an improved meixnerite product from a transition alumina and an activated magnesia which has not been dead-burned or overly calcined.
On a preferred basis, synthetic meixnerite can be made from fairy inexpensive and readily available reactants by this process thus making it suitable for the commercial scale production of meixnerite and meixnerite-like materials. It is yet another objective to provide an improved meixnerite manufacturing process which outperforms (in terms of yield) other known methods including those involving magnesium carbonates, magnesium hydroxides and/or aluminum hydroxides.